1. Field of the Invention
The present invention relates to a fixing device, more particularly to a belt fixing device having a so-called endless belt, and an image forming apparatus such as a copying machine, a facsimile or a printer having the same.
2. Discussion of the Background
An image forming apparatus such as a copying machine, a facsimile, a printer or a printing machine generally includes a fixing device for fixing a toner image on a sheet-like medium, on which the toner image to be fixed has been formed.
With regard to the fixing device, the related art has presented a heat roller pair type fixing device, and a so-called belt fixing device. The former includes a rotary-driven heating roller which has a heat source provided therein, and a pressing roller pressed into contact with the heating roller and rotated therewith. This device is designed to pass a sheet through a nip portion formed by the two rollers, based on the rotations of the rollers, and then fix a toner image to a medium by applying heat and pressure. The belt fixing device includes an endless fixing belt for conveying a sheet-like medium on which a toner image is to be fixed, heating and fixing rollers having the fixing belt wound thereon, a pressing roller disposed opposite to the fixing roller via the fixing belt, and so on.
For the belt fixing device, to provide a quantity of heat necessary for fixing a toner image on the sheet-like medium during fixing, a device having heaters provided in the heating and pressing rollers has been available. However, although the heater is provided in the pressing roller, it is difficult to increase the temperature of the fixing roller by the heater of the pressing roller. This is because the fixing roller, which forms a nip portion with the pressing roller, is generally made of a material such as silicon rubber or the like having a large heat capacity. Consequently, as recognized by the present inventors, when the fixing belt is rotated during fixing, if a portion of the fixing belt heated by a heating roller is abutted on the low-temperature fixing roller, the heat is absorbed by the fixing roller to steeply lower the temperature of the fixing belt. The quantity of heat necessary for fixing then becomes insufficient causing a fixing failure. This problem is particularly conspicuous when the temperatures of the fixing roller and the fixing belt are lowered at the time of rising temperature after the power source of the image forming apparatus, is turned on or with the passage of time after the previous image formation.
To solve the above-described problem, the fixing belt may be subjected to preparatory rotation, i.e., pre-rotation, for a long time, before the start of fixing. This allows the fixing operation to be started after a sufficient increase in the temperature of the fixing roller. However, this pre-rotation takes too much time before the start of the fixing, which runs counter to a recent tendency to place importance on the shortening of image forming time.
In addition, to shorten the image forming time, efforts have been made to increase a speed for conveying a sheet-like medium. Such an increase in the conveying speed of the sheet-like medium is particularly important in the case of an image forming apparatus designed for color image formation, because it takes relatively longer time to form a color toner image. Thus, an increase in the conveying speed of the fixing belt has become important in the belt fixing device provided in the color image forming apparatus. However, the increase in the conveying speed of the fixing belt results in the shortened staying time of the sheet-like medium in the nip portion. Therefore, in the case of increasing the conveying speed of the sheet-like medium, it is more difficult to provide the quantity of heat necessary for fixing an image thereby causing a fixing failure. Moreover, in color image formation, desired high and stable glossiness of the image cannot be obtained.
An object of the present invention is to provide a belt fixing device capable of preventing a reduction in the temperature of a fixing belt caused by its abutment on a low-temperature fixing roller. Another object is to provide an image forming apparatus equipped with such a fixing device.
According to a preferred embodiment of the present invention, a fixing device includes: an endless fixing belt configured to convey a sheet-like medium on which a toner image is to be fixed; heating and fixing rollers configured to have said fixing belt wound thereon; a pressing roller configured to be disposed opposite to said fixing roller via said fixing belt; and wherein said fixing device further comprises first to third heaters configured to be provided respectively in said heating, pressing and fixing rollers; first to third voltage applying devices configured to apply voltages respectively to said first to third heaters; and a nip portion configured to perform the fixing only by pressure contact between said fixing and pressing rollers via said fixing belt.
Further, said third voltage applying device is identical to said first or second voltage applying device.
Further, the fixing device further includes: first temperature detecting device configured to be disposed opposite to said heating roller via said fixing belt to detect a temperature of said heating roller; and second temperature detecting device configured to be disposed opposite to said pressing roller to detect a temperature of said pressing roller, wherein on the basis of a temperature detected by said first or second temperature detecting device, said third heater and said first or second heater are simultaneously controlled by said identical voltage applying device.
According to another preferred embodiment of the present invention, a fixing device includes: an endless fixing belt configured to convey a sheet-like medium on which a toner image is to be fixed; heating and fixing rollers configured to have said fixing belt wound thereon; a pressing roller configured to be disposed opposite to said fixing roller via said fixing belt; and wherein said fixing device further comprises first to third heaters configured to be provided respectively in said heating, pressing and fixing rollers, and first to third voltage applying devices configured to apply voltages respectively to said first to third heaters; and said third voltage applying device is identical to said first or second voltage applying device.
Further, the fixing device further includes: first temperature detecting device configured to be disposed opposite to said heating roller via said fixing belt to detect a temperature of said heating roller; and second temperature detecting device configured to be disposed opposite to said pressing roller to detect a temperature of said pressing roller, wherein on the basis of a temperature detected by said first or second temperature detecting device, said third heater and said first or second heater are simultaneously controlled by said identical voltage applying device.
According to another preferred embodiment of the present invention, a fixing device includes: an endless fixing belt configured to convey a sheet-like medium, on which a toner image is to be fixed; heating and fixing rollers configured to have said fixing belt wound thereon; a pressing roller configured to be disposed opposite to said fixing roller via said fixing belt; and wherein said fixing device further comprises first to third heaters configured to be provided respectively in said heating, pressing and fixing rollers, and first to third voltage applying devices configured to apply voltages respectively to said first to third heaters; said third voltage applying device is identical to said first or second voltage applying device; temperature detecting device is provided in a roller having a higher thermal responsiveness; and on the basis of a temperature detected by said temperature detecting device, said third heater and said first or second heater are simultaneously controlled by said identical voltage applying device.
Further, said heaters having voltages applied thereto by said identical voltage applying device are connected to the voltage applying device in parallel.
Further, outputs W1, W2 and W3 of said first to third heaters are set to satisfy a relation of W1 greater than W2 greater than W3.
According to another preferred embodiment of the present invention, an image forming apparatus includes: an image forming device configured to form an toner image on a photosensitive member and to transfer the toner image on a sheet-like medium; and a fixing device configured to fix the toner image on the sheet-like medium, which is specified in claim 1, 4 or 6.
Further, the image forming apparatus is capable of forming color images.
According to another preferred embodiment of the present invention, a fixing method includes: conveying a sheet-like medium on which a toner image is to be fixed by an endless fixing belt; applying voltages respectively to first to third heaters by first to third voltage applying devices; heating said first to third heaters provided respectively in heating, pressing and fixing rollers; and performing the fixing only by pressure contact between said fixing and pressing rollers via said fixing belt by a nip portion.
According to another preferred embodiment of the present invention, a fixing method includes: conveying a sheet-like medium on which a toner image is to be fixed by an endless fixing belt; applying voltages respectively to first to third heaters by first to third voltage applying devices; heating said first to third heaters provided respectively in heating, pressing and fixing rollers; and wherein said third voltage applying device is identical to said first or second voltage applying device.
According to another preferred embodiment of the present invention, a fixing method includes: conveying a sheet-like medium on which a toner image is to be fixed by an endless fixing belt; applying voltages respectively to first to third heaters by first to third voltage applying devices; heating said first to third heaters provided respectively in heating, pressing and fixing rollers; and wherein said third voltage applying device is identical to said first or second voltage applying device; a temperature detecting device is provided in a roller having a higher thermal responsiveness; and on the basis of a temperature detected by said temperature detecting device, said third heater and said first or second heater are simultaneously controlled by said identical voltage applying device.